the
hanging-drop slide. Remove the bell glass altogether and press the
cover-slip firmly on to the cell.

8. Either incubate and examine at definite intervals, or observe
continuously with the microscope, using a warm stage if necessary (Fig.
53).

(b) ~Solid Media.~--Observing precisely similar technique, a few drops of
liquefied gelatine or agar from the second dilution tube may be run over
the surface of the sterile cover-slip and a hanging-drop plate
cultivation thereby prepared.

This method is extremely useful in connection with the study of yeasts,
when the circular cell on the hanging-drop slide should be replaced by a
rectangular cell some 38 by 19 mm., and the gelatine spread over a
cover-slip of similar size. After sealing down the preparation, the
upper surface of the cover-slip may be ruled into squares by the aid of
the grease pencil or a writing diamond and numbered to facilitate the
subsequent identification of the colonies which are observed to develop
from solitary germs.

~Hanging-block Culture~ (Hill).--

_Apparatus required_: As for hanging-drop cultivation with the addition
of a scalpel.

Carry out the method as far as possible under cover of a bell glass, to
avoid aerial contamination.

1. Liquefy a tube of nutrient agar (or gelatine) and pour into a Petri
dish to the depth of about 4 mm. and allow to set.

2. With a sharp scalpel cut out a block some 8 mm. square, from the
entire thickness of the agar layer.

3. Raise the agar block on the blade of the scalpel and transfer it,
under side down, to the centre of a sterile slide.

4. Spread a drop of fluid cultivation (or an emulsion of growth from a
solid medium) over the upper surface of the agar block as if making a
cover-slip film.

5. Place the slide and block covered by the bell glass in the incubator
at 37 deg. C. for ten minutes to dry slightly.

6. Take a clean dry sterile cover-slip in a pair of forceps, and with
the help of a second pair of forceps lower it carefully on the
inoculated surface of the agar (avoiding air bubbles), so as to leave a
clear margin of cover-slip overlapping the agar block.

7. Invert the preparation and with the blade of the scalpel remove the
slide from the agar block.

8. With a platinum loop run a drop or two of melted agar around the
edges of the block. This solidifies at once and seals the block to the
cover-slip.

9. Prepare a sterile hanging-drop slide, and smear hard vaseline or
m